Parity bit generator



April 3, 1962 J. H. VELDKAMP 3,028,588

PARITY BIT GENERATOR Filed July 20, 1959 2 Sheets-Sheet 1 i 28 INVENTOR.A MP 3 HG H JEAN H VELD A lax WWW ATTORNIYJ April 1962 J. H. VELDKAMP3,028,588

PARITY BIT GENERATOR Filed July 20, 1959 2 Sheets-Sheet 2 FIG. 8

FIG. I2

MEAN H. VEL DKAMP m BY United States Patent 3,028,588 PARITY BITGENERATOR Jean H. Veldkamp, Hengelo (Overijsel), Netherlands, as-

siguor to N.V. Hollandse Signaalapparaten, Hengelo (Overijsel),Netherlands Filed July 20, 1959, Ser. No. 828,319 Claims priority,application Great Britain July 21, 1958 17 Claims. (Cl. 340-345) Thisinvention relates to an electro-magnetic step-bystep mechanism, and inparticular to such a mechanism which drives a display unit, a memorysystem or a similar system.

It is an object of the invention to increase the speed of operation ofsuch a mechanism as well as to reduce its volume. It permits such areduction of the crosssection of a display unit that a large number ofsuch units may be housed in a small display board. The width of apractical embodiment of a display unit according to the invention wasonly so that 25 units of this type, each of them showing a letter or afigure, can be mounted on a line having a length of 1 ft. only. Theheight of the said unit was so that, allowing for supporting platesbetween the units, 20 lines can be housed in a panel with a height of 1ft. The setting speed of the said unit was, moreover, high. It has beenused at speeds of 120 steps per second, and speeds of 50 to 60 steps persecond could be obtained for a very substantial period of useful life.

- In the step-by-step mechanism according to the invention the plungercore of the driving electro-magnet drives a rotatable drum by means of aresilient drawing pawl and a resilient pushing pawl, the driven end ofeach pawl being fixedly mounted to an element connected to or formingpart of the plunger core, which pawls cooperate with at least oneratchet wheel with inside teeth, countersunk into the said rotatabledrum. 7

Preferably the pawls are made from resilient wire, such as steel wire.

In a preferred formof the step-by-step mechanism according to theinvention the drum is provided with two countersunk ratchet wheels, oneat each end, each pawl cooperating with its own ratchet wheel.

Furthermore, in a preferred form of the step-by-step mechanism accordingto the invention, the drum is provided with pins or sprockets,by meansof which it drives an endless belt.

In a suitably built magnetic system with a slideable core it is possibleto obtain a high ratio of the mechanical energy supplied by the systemto the volume of the system, as well as a high ratio of said mechanicalenergy'to the weight of the system whereas the force exerted on thearmature, i.e. on the slideable core, shows only relatively smallvariations due to the change in position of this core during its stroke.This permits a small magnetic driving system to be used for driving thestep-by-step mecharusm.

The ratchet wheel or wheels being countersunk into the rotatable drum,no part of the length of the drum need be reserved for the ratchet orratchets, and this permits the application of a drum with a length notlarger than is necessary for carrying the signs to be displayed or fordriving and supporting the belt to be driven. The application ofresilient pawls, especially of pawls made of resilient wire, permits thebuilding of a unit the dimension of which in the direction of the axisof the rotatable drum is only slightly larger than the length of thisdrum. The application of fixedly mounted resilient pawls makes hinges orrotatable connections for the pawls which, as a rule, are subject toserious wear in such small and rapidly operating apparatus, superfluous.

Recapitulating, it can be stated that the invention per- 3,028,588Patented Apr. 3, 1962 'ice mits the building of light and smallstep-by-step mechanisms which, because of the small weight and inertiaof the moving parts, can be used at very high stepping speeds.

In one form of the mechanism a single energizing coil is wound on aspool consisting of a non-magnetic tubular element and two flanges ofsoft iron mounted on the tubular element. An outside yoke connects theseflanges. The movable plunger core enters the tubular element at one end,whilst a fixed core enters it from the other end. Preferably one of thecores has a conical end at the side facing the other core whichpossesses a correspondingly shaped conical recess. A spring acts on theplunger core, causing it to rest against a stop in a position in whichit has the greatest distance from the fixed core. If the coil is excitedthe plunger is pulled into the tubular element until it reaches anotherstop restricting the motion to such an extent that a small air gapremains between the plunger and the fixed core.

In another form of the invention the driving magnet system possesses twowindings wound on separate coaxial spools which are adjacently mountedin a direct line, each of them consisting of a non-magnetic tubularelement and two soft iron flanges connected by an outside yoke.Preferably both spools possess a common tubular element and one commonflange at the adjacent ends. Fixed cores enter the tubular element orelements at the non-adjacent ends, and the movable core is situated inbetween these fixed cores. The ends of the cores facing each other arepreferably conically shaped as described in the case of the firstembodiment. No springs act on the movable core, this core being shiftedin the one or the other direction by energizing the one or the othercoil. In a preferred form of this driving magnet the core is coupled bymeans of a rod passing through a bore in one of the fixed cores to theelement to which the pawls are fixedly mounted.

The two pawls are preferably made of resilient wire and are fixedlymounted to the part carryingv them and owing to their elasticity areable to swing.

If the drum driven by the pawls is the last driven object and thestep-by-step mechanism is used as a dis play unit, this drum carries thesigns to be displayed. If the drum drives a belt and the mechanism isused as a display unit, the signs to be displayed are represented onthis belt. In certain embodiments the two pawls co-' operate with theopposite halves of a single ratchet wheel, but if high requirements asto operating speed and small dimensions must be met, two ratchet wheelsare used. Preferably these ratchet wheels are relatively displacedthrough an angle which may be equal to half the cir-. cular pitch ornearly that value, each pawl cooperating with its own ratchet. It willbe shown below that in this case the stresses in the pawls can remainsmaller.

In various applications of the step-by-step mechanism it is desirablefor the position of the element driven by the magnet to be signalledback. The number of pulses applied to the magnet coil is not aninfallible measure for the position reached, because a step-by-stepmechanism may get out of step. Consequently certainty as to the positionreached can only be obtained by signalling back. A certain amount ofspace is, however, required for the mounting of the elements controllingthe signalling back and the volume of the step-by-step mechanism isreduced to such an extent by the application of the invention, that thisspace cannot be found without special measures. According to a specialform of the invention these measures consist in mounting the saidelements controlling the signalling back circuits in the space enclosedby the belt.

In one embodiment according to the invention the belt driven by the drumpossesses for this purpose at least one track passing under a set ofbrushes mounted in the space enclosed by the belt by means of whichbrushes electrical circuits can be closed in various permutationsthrough suitable openings or recesses in the track, for the purpose ofsignalling back the position reached by the stepping mechanism. Thebrushes are mounted in a line so as to rest on the track of the belt,whilst electrical contact is established between a brush and a contactstrip or between a brush and one contact element out of a set of contactelements mounted in a line at the opposite side of the belt, if anopening or recess in the track of the belt has reached the positionbetween the said brush and the contact strip or the contact element. Theopenings or recesses in the belt are situated in such a way that theypermit the contact between a brush and a contact element whilst thestepping mechanism is in one of its positions of rest. If the steppingmechanism is a display unit a sign is displayed in such a position ofrest. The recesses or openings in the belt are arranged so as to form ashifting code, indicating the position reached and the sign shown by thedisplay unit. In normal use the stepping motor is fed by a pulsegenerator until a suitable control circuit establishes thatsignalling-back circuits in a desired code combination are closed by thebrushes. Then the circuit between the stepping mechanism and the pulsegenerator is broken, causing the belt to remain at rest in the desiredposition.

The stepping mechanism can also be used as a memory which can be readout by means of the circuits through the various brushes. If two tracksprovided with recesses or openings, cooperating with a set of brushesare present, the step-by-step mechanism can be applied as a codeconverter.

In another embodiment according to the invention, perinitting the use ofsignalling back contacts, the belt possesses at least one track on whicha feeler or a set of feelers, mounted in the space enclosed by the belt,rests. Such a track is provided either with projections or with openingsor recesses, causing the positions of the feelers to be dependent on theposition of the belt. The feelers close or open electrical contacts insignalling back circuits, depending on the positions reached by themunder the control of the belt.

This embodiment can also be applied as a memory mechanism or as codeconverter.

The way of mounting the contact devices for signalling back describedabove, permitting these devices to be confined in the small spaceavailable, requires the use of a shifting code. A shifting code can,however, not be made self-checking by the simple means applied for thispurpose Withother codes, such as arranging the code so as to make thenumber of elements of one type always even or always uneven.Nevertheless the application of such a code is desirable in order toprevent imperfect contacts in the signalling back circuits fromproducing mutilated signals. According to a special form of theinvention the application of a self-checking code is made possible bymounting a special contact device in the space enclosed by the belt,resting on a separate track of the belt in which such openings,recesses, or projections are present that the code furnished by allcontact devices becomes selfchecking.

Step-by-step mechanisms according to the invention, applied in displayunits according to the invention, will now be described with referenceto the drawings.

FIG. 1 shows a cross section of a driving magnet system of astep-by-s'tep mechanism, and

FIG. 2 partly a front view and partly a cross section according to theline I-I in FIG. 1 of this magnet system.

FIG. 3 shows a step-by-step motor according to the invention. v

FIG. 4 shows the step-by-step motor according to FIG. 3 seen from theopposite side.

FIG. 5 shows a detail of the step-by-step motor of FIG. 3.

FIG. 6 shows the cooperation of the pawls and ratchet wheels.

FIGS. 7 and 8 show a complete display unit according to the invention.

FIGS. 9 and 10 show the brushes and contacts of a display unit.

FIG. 11 shows a belt for the display unit.

FIG. 12 shows a driving magnet system with two coils.

FIGS. 13 and 14 show the cooperation of a belt and a set of feeler armsfor the purpose of signalling back.

FIG. 1 shows the cross section of the driving magnet system of astep-by-step mechanism according to the invention. The winding 2 of thedriving magnet of this system is wound on a spool consisting of anon-magnetic tubular element 20, provided with soft-iron flanges 3 and17. The tubular element may be provided with a longitudinal slot inorder to avoid eddy currents, but if the magnet is fed by direct currentpulses with frequencies up to c./s. such a slot is not absolutelynecessary. A fixed soft iron .core 18 of suitable cylindrical shape,provided with a conical end 19, is mounted in the tubular element 20,and a movable cylindrical core 21 can be shifted longitudinally in thetubular element and possesses a conical recess 22, the shape of whichcorresponds to the shape of the cone 19. The core 18 can be shiftedlongitudinally in the tubular element from the purpose of adjusting theair gap between the cores in the position of rest. It is fixed by aclamping screw (not shown in the figure). The cylindrical part of themovable core glides this core during its motion in the tubular elementand should be of at least such a length that jamming and wear of thecore is avoided. The object of the conical parts is to reduce themagnetic resistance of the air gap and, by making the recess in themovable core instead of in the fixed core, the weight of the movablecore is reduced without reducing the length of the guiding surface. Thisis important because it permits the core to move at a higher frequencywith the same inertia resistance. The magnetic circuit of the coil 2,which passes through the flange 3, the movable core 21, the fixed core18, and the flange 17, is closed through flat iron yoke parts 1 and 16,which are screwed to the flanges 3 and 17. The coil 2, the flange 3, andthe yoke elements 1 and 16 may also beseen in FIGS. 3 and 4, to whichthe following description also refers. A U-shaped frame element with thearms 9 and 14 is mounted onto the flange 3 by means of screws such as 15and spacers such as 4. A square frame 5, preferably made of materialwith low rate of wear, such as nylon or a similar polyam-ide, is screwedto the outer end of the movable core 21. The central part of theU-shaped frame, which is parallel to the flange 3 of the coil, passesthrough the opening of this square frame. A spring 6 is mounted betweenthe right hand side of the square frame and the central part of theU-shaped frame, which spring is kept in place by cylindrical stubs suchas 25, which are present on the right hand part of the square frame andon the centralpart of the U-shaped frame. The force exerted by thisspring 6 causes the core 21 to be moved to the right to such an extentthat the left hand side of the square frame rests against the centralpart of the U-shaped frame. If the coil 2 of the step-by-step mechanismis excited, the movable core 21 is moved to the left against thepressure of the spring 6, causing the length of the air gap between theconical parts of the fixed and the movable core to be reduced to a fewhundredths of a millimeter. The motion of the core is restricted by thesquare frame when this frame reaches the flange 3. The inside surface ofthe tubular element 20 is smooth, and in order to reduce wear themovable core 21 is covered by a thin layer of nylon or a similarpolyamide or by a thin layer consisting of a mixture of such a polyamideand a small percentage of molybdenum di-sulfide (e.g. molycote). Thislayer is made by dipping the heated core into a container containingfine nylon or a similar polyamide powder, or a mixture of fine nylon ora similar polyamide powder and fine molybdenum di-sulfide powder, thispowder or mixture being kept in motion, more or less in suspension, by astream of compressed air. In order to promote the adherence of thepolyamide to the core the core possesses a rough surface. A shaft 12 ismounted in the U-shaped' frame and carries a sprocket wheel in the shapeof a drum 11, preferably made of nylon or a similar polyamide, and whichis freely rotatable around the said shaft. This dnlrn carries at bothsides .a ratchet wheel. In order to reduce the axial length of the drumrequired for a given width of the belt to be driven by it, the ratchetwheels possess inside teeth and are counten sunk in the drum, such asshown for the ratchet wheel in FIG. 3. The square frame 5 carries twopawls 8 and 13, which are made out of one piece of steel wire and fixedto the square frame 5. It would be possible to fix the pawls to theframe by means of screws, but in the example shown that part of thesteel wire of the pawls which is to be fixed, is directly bedded in thepolyamide. In this embodiment the free parts of the pawls are for asubstantial part situated in cylindrical recesses 7 in the square frame,in which recesses they are free to swing, so that the length of the pawlrequired in connection with the amplitude of the traversely swingingpawl motion, may be housed in a shorter unit. The pawls 8 and 13 arebent at their ends, so as to possess end parts which are parallel to theshaft 12. The pawl 13 rests on the inside toothing of the ratchet wheel10, whilst the end of the pawl 8 rests on the inside toothing of theother ratchet wheel 23. As may be derived from FIG. 3 and FIG. 4, thepawl 13 rests on the lower half of its ratchet wheel, whilst the pawl 8rests on the upper half of its ratchet wheel. The ratchet wheels are soshaped that they drive the drum 11 in an anticlockwise direction. Thecooperation of the two pawls and the two internal ratchet wheels may bederived from FIG. 6, in which the front ratchet 10 and the back ratchet23 as well as the front pawl 13 and the back pawl 8 are schematicallyshown. It is assumed that the two ratchet wheels are coupled so as torotate together. When the movable core is drawn to the left by themagnetic field, the back pawl 8, operating as a drawing pawl andcooperating with the steep side of a tooth of the back ratchet wheel 23,causes this ratchet wheel and the front ratchet wheel 10 to rotate. Thestroke of the core and the pawls corresponds to a rotation of theratchet wheels through an angle which is half or about half the angle ofrotation of the ratchet wheels caused by one complete to and fro motionof the core, i.e about half the circular pitch of the ratchet wheels.The ratchet wheels shown possess six teeth, so that the stroke of thecore must correspond to about 30 ratchet wheel motion.

It is desirable to restrict the swinging motion of the driving pawls,because at the high frequency of this motion fatigue fractures at theroot of these pawls may very well occur. The amplitude of this swingingmotion is determined by the extreme positions of the end of the pawl,cooperating with the ratchet occurring during its complete to and fromotion. Depending on the positions of the points of reversal of thismotion, extreme positions of the free end of the pawl may occur atvarious points of the path of the end of the pawl. These points are:

(1) The point where the said end of the pawl is in the plane of symmetrythrough the axis of rotation of .the ratchet wheel perpendicular to thedirection of motion of the core of the magnet and where it rests on thefull depth circle of the ratchet wheel (point A in FIG.

6). The said end of the pawl can only be in this point when it passes,or at any rate reaches this symmetry plane during its motion.

(2) The point where the end of the pawl reaches the full depth circleafter having fallen from the tip of a tooth only in the case, however,of the said end not reaching or passing the said symmetry plane.

(3) The point where the end of the pawl passes over the tip of a tooth(point B in FIG. 6).

(4) The point C where the end of the pawl reverses its motion afterhaving driven the ratchet wheel. This may be a point where an extremeposition occurs, but, as a rule, it is not.

(5) The point which presents itself when the end of the pawl, whilstmoving over a non steep face of a tooth, is lifted higher than it is atthe moment at which it passes over the tip of that tooth. It is,however, undesirable to lift the pawl higher than is necessary forpassing the tip of the tooth because bending stresses and pawl pressureincrease when the pawl is lifted, and high pawl pressures cause wear.Consequently it is desirable to shape the non steep face of the tooth insuch a way as to lift the pawl no higher than is necessary for passingthe tip. Such a shape is feasible in practically any case. If the pointsof reversal of the motion of the driving end of the pawl are situated onboth sides of the symmetry plane mentioned above, then at any rate thepawl end will pass through the point A, where in this case it willoccupy an extreme position. The other extreme position will either besituated in the point B or in the point C. In practical forms of thestep-bystep motor according to the invention the number of teeth of theratchet wheel will not be less than five, because otherwise theamplitude of the motion of the pawl will become too large. In the formdescribed the ratchet wheel has six teeth, in which case every one ofthe two pawls drives its ratchet through an angle of 30". In these casesthe pawl will be lifted less high when its end is at point C than whenthe end is at point B, if the teeth are high enough to cooperate with apawl of sufficient strength, e.g. with a diameter of at least 0.6 mm.,even if the stroke during which the pawl drives the ratchet wheel startsin the said symmetry plane. In the practical embodiment described thediameter of the full depth-circle of the ratchet wheel is 9 or 10millimeters and the depth of the teeth 1 mm, the number of teeth beingsix. It can then easily be shown that the end of the pawl is lifted lesshigh at point C than at point B. If the points of reversal of the motionof the end of the pawl are situated on both sides of the symmetry plane,the lower extreme position remains A, but the pawl must be lifted higherto pass over the tip of the tooth, the position in which this takesplace being the position B, for instance. It is, therefore, clear thatin order to reduce the amplitude of the swinging motion of the pawl themotion of its end should be preferably performed at that side of thesymmetry plane where the driving stroke ends, the pawl passing over theteeth at the point of reversal nearest to the symmetry plane. If howeverthe end of the pawl passes the symmetry plane during the driving stroke,but only a small part of the stroke is performed before this end reachesthe said symmetry plane, this will have no serious consequences. Even ifthe stroke is situated symmetrically with respect to the said symmetryplane the swing of the pawls is only increased by 10%, but the bendingstresses in the root of the pawl increase rapidly if a larger part ofthe stroke is performed before the end of the pawl passes the symmetryplane, reaching in the embodiment described of the stress occurringduring the most favourably situated stroke if the driving stroke iscompleted before the symmetry plane is reached. If, in the case of aratchet with six teeth, the driving stroke of the end of the pawl beginsin the symmetry plane, the pawl will, during the return stroke, belifted by the face of the tooth in order to reach the height requiredfor passing over the tip of the tooth. As a rule this is undesirable,because at the very high operating speed this lifting operation causesrelatively large forces to be ex- 'full depth circle.

:straight line.

the pushing pawl as well as for the drawing pawl. One

7 erted on the face of the tooth. The point B at which the pawl passesover the tip of the tooth may, however, be situated at the same heightas the point C, reached 'at the end of the active stroke. For thispurpose the end of the pawl should never pass the said symmetry planeand should start its active stroke not in, but at a small distance fromthis plane. In the case of e.g. a wheel with an outercircle diameter of9 mm. and six teeth with a depth of 1 mm. the bending'of the pawl is thesame when its end is in the points C and B if theleft hand reversalpoint of the motion is situated at 9 or 10 from the symmetry plane. Ithas been mentioned above that the shape of the non steep face of thetooth must be such that it does not lift the pawl higher during itsmotion over this face than near its tip. This may be approximatelyrealised by using teeth the non steep face of Whichis cylindricallyshaped with a diameter which is slightly larger than the diameter of theIn the example shown for instance, this face of the tooth is acylindrical surface with a the pawl end passes over the tip of a toothis situated ,at 10 from the symmetry plane the return-motion of theendof the pawl is practically a motion along a The above considerationshold true for of the pawls is a drawing pawl and the other one a pushingpawl, and if the pushing pawl operates in a sector corresponding to halfthe circular pitch on the one side of the symmetry plane the drawingpawl must operate in a corresponding sector on the other side of thesymmetry plane and diametrically situated with re- :spect to the othersector. in turn. In order to obtain the smallest bending stresses "thedrivingshould start at two points which are on the Both ratchets must bedriven same diameter. The driving must be continued through 'half thecircular pitch. Consequently the smallest stresses will result if bothratchets are relatively displaced through an angle of about half thecircular pitch of the ratchet wheels. The length of the pawls mustditfer to such an extent that when one of the pawls has reached the endof its operative stroke the other pawl has reached the position in whichit passes over the tip of a tooth. The

'completeunit willnow be described with'reference to the FIGURES 7, 8,9, and 10. The unit is housed in a sheet steel U-shaped housing 31, inwhich the stepby-step motor with its winding 2 is mounted by means ofcounter sunk screws, screwed in the flanges 3 and 17. A brush support ofinsulating material indicated by the reference numbers 27 and 42 isfixed in a similar way in the said housing. It possesses a middle partof reduced cross section on which, by means of screws such as 33, abrush plate 29 is fixed. This carries six brushes 30 at the one side ofthe part of insulating material and 37. The piece of plate material ismounted on a tubu-.

lar element 36 which is supported in a cylindrical opening in the partof insulating material 42. The tubular element contains a spring 34which keeps the belt 20 taut in all conditions. The direction of theshaft 39 is determined by a slot in the part of insulating material 42,in which slot the part of plate material 37 narrowly fits. A part of thebelt is shown in FIG. 11. Suitable signs such as letters or figures areprinted between the openings 60. When the belt is driven by thestep-by-step motor it slides 'under the brushes of the "brush holder.

"strip. -In this channel the wire is situated betweenthe strip and thechannel shaped housing. In this;way--the "strip of insulating material40 prevents damage to the -belt by the said wire should it becomedislodged.

'In' the trajectory of these brushes near both edges of the belt,suitable recesses such as '58 and 59 are present. If the step-by-stepmotor is at rest the position of the belt is'such that either a recessor a tooth of the belt will be situated under each brush of the brushholder, causing electric circuits through these brushes and contacts,such as 46, to be closed or opened. The recesses and thete'ethcontrolling the circuits through the siX brushes 30 are situated so asto form a six bit shifting code denoting the sign situated on the pulley38, and shown through the top opening of the unit. A suitableconstruction'of the contacts may be derived from the FIGURES 7 and 10. Aplate of insulating material 43 is mounted in the housing by means ofcounter sunk screws such as 45 and 55. This partpossesses a recess 54permitting the passage of the sprockets 24 of the drum 11. It possesses,moreover, a channel 48, provided with a row of openings, such as 47,which are situated between and at both sides of the brushes 30 when thestrip of insulating material is mounted in the housing. Small strips ofcontact material to which a thin insulated wire is soldered, are bentaround the dams of insulating material between the said openings 47 soas to form contacts 46 able to co-operate with the brushes. In asimilarway a contact '56 is mounted in order to cooperate with thesingle brush 32at the other side of the belt. The seven wires leading tothe said contacts pass through the channel 48 and through the flatchannel at the right hand side-of the step-by-step motor to acontactiplug 52 provided 'Wlth contact pins 53 mounted in the bottomside of the housing. Each of said wires and also each of the wiresfeeding coil 2 is soldered to a contact pin in said plug. The

brush plate 21 is connected toa contact pin 48 by means of a wire whichpasses through an opening in the strip of insulating material 40 andthrough a channel in this The setting of the unit is performed in thefollowing way. A code indication of the sign to-be shown is received ina suitable register. After the reception of this indication a comparisonsystem comparing the code circuits closed by the brushes 30 with thecode indication received is made operative. If these indications are notthe same a pulse circuit for the driving motor is closed, causing thebelt 28-to be driven and the comparison system to be made inoperative.Between-stepping pulses the comparison system is temporarily caused tobe operative, and'as soon as the'code indication given by the circuitsthrough'the brushes and caused by the recesses and teeth of the belt,which code indication denotes the sign shown, is the same as the codeindication received by the register, the step-by-step'circuit for thestep-by-step motor is broken bythe comparison system; afterwhich theunit remains at rest. The number of brushes 30 is determined by thenumber of signs for which the unit can be used. In the example describedthe belt carries 40 signs, for which a six bit binary code is mounted,it is necessary to take into account the pos sibility that the contactbetween a brush and its contact may not be established when a recess isunder this brush,

because of dirt either on the brush or on the contact.

This may result either in the showing of a Wrong sign or in continuousrunning of the unit. As continuous running shows the unit to bedefective no false indication will result from it, butin most cases thefailure of a brush to contact its contact-element will result in a wrongsign being-shown by the unit. This may be conbe mounted at the otherside of the brush holder.

sidered as highly undesirable. It might be prevented by using aself-checking code e.g. a code in which the number of elements of onetype is always even or always uneven. As the code provided by thebrushes 30 is necessarily a shifting code it is impossible or at anyrate requires a large number of brushes to arrange this code in such away that it is self-checking. According to the invention, however, aself-checking code may be obtained by mounting one single brush in sucha way that it rests on the belt on a second track, for instance near theother edge, because the element provided by this brush is completelyindependent of the shifting code and may for instance, always beselected in such a way that it makes the total number of elements of thetype taken into consideration for the self-checking, either even oruneven as required.

The housing of the unit is closed by a lid 61 (FIG. 8) which narrowlyfits between the edges of the channel shaped housing and is fixed to theunit by means of counter sunk screws, two of them passing through theopenings 26 and 44 to opening provided with screw thread in the backwall of the channel shaped housing and two screwed in the openings 51and 50 in the flanges of the coil of the step-by-step unit. A plate ofinsulating material 41 is situated under the lid.

The speed of operation being high, in some cases the belt may show atendency to run on as a result of the inertia of the various movingparts such as the drum and the pulley after the step-by-step motor hasfinished its operation. This is undesirable because it results in theshowing of a mutilated sign. If a unit is to be used at very high speedsspecial measures according to the invention are taken in order toprevent this prolonged rotation. For this purpose a stop for one of thepawls is arranged which causes this pawl to be pressed against theratchet when it has reached the position of rest. FIG. shows an exampleof such a stop. It consists of a lip 57 on the supporting arm 14- of theU-shaped frame which causes the pawl to be pressed in an outwarddirection when it has reached its position of rest. Such a support forthe pawl can only be arranged for that pawl which has completed itsactive stroke when the core reaches its position of rest and which,consequently, does not pass over the tip of a tooth at the end of thisstroke. If this pawl is lifted by the ratchet during its nonoperativestroke, as has been described above for a certain situation of theposition of rest, the stop must not be extended in the direction of themagnet so as to permit this lifting. If the situation of the position ofrest is such that the pawl is not lifted during its return stroke thereare no restrictions for this stop. In this case it may consist of anextended central hub of the sprocket drum. The pawl wire must then, as arule, be bent near its end so that it can rest against the hub in theposition of rest of the pawl, as shown by dotted lines in FIG. 6.

The mechanism according to the invention can also be used as amechanical memory for a code indication. In this case the application ofa self-checking code is important in order to ensure a faultlessoperation.

Instead of one brush a larger number of brushes can A second completeset of brushes, able to give a code indication can even be applied, inwhich case the unit can be used as a code converter. On the other hand,if no self-checking code is required, a single set of brushes, restingon one side of the belt, suffices.

A driving magnet system with two coils will now be described withreference to FIG. 12. The two coils, 63 and 68, are Wound on a spoolwith three flanges. This spool consists of a non-magnetic tubularelement 64, two soft iron end flanges 62 and 71, and one central flangemade of soft iron, 66. Fixed iron cores 78 and 69 enter the tubularelement, one at each end. Both cores are provided with conical ends. Aslideable soft iron core 65, provided with recesses corresponding to theconical ends of the fixed cores, is situated between these fixed cores.The magnetic circuits for both coils, each of them passing through anend flange, a fixed core, a part of the movable core, and the centralflange, are closed by soft iron yoke elements, such as 67 and 77,screwed to the flanges. If coil 63 is energized, the slideable core isshifted to the left, if coil 68 is energized, the movable core isshifted to the right. The movable core is mounted on a slender rod 70 ofsmall diameter, for instance by means of screw thread cut in a centralpart of this rod, which has a slightly larger diameter. This rod passesthrough bores in the fixed cores. It carries adjustable stop elementssuch as nuts 76 and 79 and lock nuts. Such a stop limits the motion ofthe core by abutting against a fixed core. By means of a nut 74 a smallbar 72 is screwed to the rod 70 :and, consequently, moves with the core.The bar 72 is situated diagonally in the housing of the stepby-stepmechanism so that it cannot rotate around the axis of the rod 70. Wirepawls 73 and 75, preferably made out of one single piece of steel wire,are bedded in the bar 72. Preferably the fixed cores 69 and 78 are soarranged that they can be shifted or screwed (by means of screwthread inthe flanges and on a part which has a slightly larger diameter) inwardlyand outwardly in the flanges 71 and 62 in order to adjust the spaceavailable for, and the situation of the stroke of the movable core 65.After the fixed cores have been adjusted and fixed, for instance bymeans of set screws in the flanges, the minimum length of the two airgaps is adjusted by means of the stops 76 and 79. It is an advantage ofthis magnet system that no spring is required, so that no space need bereserved for the spring, and the magnet system need not furnish theforce for compressing this spring. Consequently it is possible for themagnetic field to be weaker, causing the magnetomotive force requiredfor the air gap as well as the leakage to be diminished so that thenumber of windings per spool can be substantially smaller than in themagnet system shown in FIG. 3. The two coils can be alternatively fedthrough oppositely directed rectifiers by one single source ofalternating current. On the other hand it is a disadvantage of this twocoil driving magnet system that it has no definite position of rest,because it remains in the position which it reached under the influenceof the coil which has been excited last.

It would be possible to apply a driving magnet system according to FIG.12 in such a way that it possesses two positions of rest, the drivenbelt or drum changing the sign shown when the core is shifted to theleft as well as when the core is shifted to the right.

The contacts in the signalling-back circuits closed by means of thebrushes 30 described with reference to FIG. 7 have a tendency to beunsafe as a result of dirt settling on the brushes or on the contacts.The signalling-back contacts of the type shown in the FIGURES 13 and 14are in this respect to be preferred. In FIG. 13 part 79 is the belt,provided with recesses such as 86. Lever shaped feelers rest on thisbelt on the track in which the recesses are present. Two of thesefeelers, namely the feeler 80 and the feeler 85, are shown in FIG. 13.The feeler 80 is a lever which is rotatable around a shaft 8 1 and issituated between two bars 87 and 88- Which, in

this construction, replace the central part of the insulating brushcarrier 27, 42 of FIG. 7. The upper end of the lever moves between twopins 82 and 84. These pins are used as contact elements. A blade spring83 is mounted to the lever and rests on the contact element 84-,

. tending to rotate the lever in an anti-clockwise direction.

The lever can nevertheless not reach the contact member 82 as long asthe lower end of the lever rests on the belt, as shown in the case ofthe lever 80. As soon, however, as a recess such as 86 in the belt issituated under the lower end of the lever, the blade spring rotates thelever to such an extent that the upper end contacts belt flexible.

1 1 the contact element 82, causing a signalling-back circuit to beclosed through the contact element 84, the blade spring 83, the lever 80and the contact element'8'2. The position then reached by the lever isshown in the figure in the case of lever 85. The right hand side of FIG.13 shows a lever as seen in the longitudinal direction of the belt.Levers, contacts and supports are centrally situated in the unit inorder to reduce the volume, but the tracks on which the levers must restare situated near the edge of the belt so that the levers have to havethe special zig-zag shape shown in the right hand side of the figure.

It is a disadvantage of the contact construction accord ing to FIG. 13as well as of the contact construction according to FIGS. 7 and 10 thatrecesses or openings must be made in the belt, causing the strength ofthis belt to be reduced. No such openings are required in the case ofthe construction shown in FIG. 14. In this construction similar contactlevers are applied as feelers,

but insteadof recesses or openings in the belt, projections are made onthe belt, which are able to lift the lower extremity of such a lever. Alever of this type opens its contact if it rests on a projection of thebelt and closes this contact if no projection is present under its lowerend. As the belt must remain flexible, the dimensions of For thispurpose, in

the projections are divided into separate parts, the dimensions of whichare sufliciently small to keep the In another embodiment of a contactsystem in astep-by-stepmechanism according to the invention only onesui'liciently small projection is used for every position of rest, inwhich a feeler should be lifted. This should not, however, influence thelength of the route along which the belt can travel without .changingthe position of a lever resting on one of its projections.

The levers are, therefore, provided with feeler parts, which areparallel to the surface of the belt whilst this part is resting on aprojection and the length of this part is such that the lever remains ina position shown in FIG. 14 in the case of the right hand lever "duringa displacement of the belt corresponding to about one complete to andfro motion of the magnet armature.

What I claim is:

1. An electromagnetic mechanism comprising a driving electromagnet witha winding enclosing a non-magnetic tubular element carrying twoferro-magnetic end flanges connected by an external ferro-rnagnetic yokesystem, a fixed ferro-magnetic core mounted in the said tubular elementso as to be partly in the opening enclosing the tubular element in oneof the end flanges, a ferro-magnetic plunger core slidable in thetubular element, a supporting element fixedly connected to said plungercore, two driving pawls, a drawing pawl and a pushing pawl, both -madeof resilient material and each of them fixedly supported at one end bythe said supporting element, a rotatable drum situated in line with thedriving magnet and with its axis of rotation traverse to the axis ofsaid magnet,

. an endless belt, carried around and in driving connection protrudinginto the countersunk ratchet wheel with which the pawl cooperates andresting by the pawls o-wn elastic force on the toothing of said wheel,either near one edge or near both edges of the belt, a longitudinaltrack provided with different parts following each other in thelongitudinal direction of the track and some of which have one givenmaterial thickness and others of which have another given materialthickness which may be a zero, a contact mechanism controlled by thebelt situated-in the space enclosed by the endless belt, a number 12 ofcontact devices-in said mechanism eachprovided with a part cooperatingwith a longitudinal trackof the belt, at least one electrical contact insuch a contact device openedand closed by the device depending on thematerial thickness of the belt in the track on the spot Where the saidpart of the contact device cooperates with the track.

2. An electromagnetic mechanism comprising a driving electromagnet witha winding enclosing a non-magnetic tubular element carryingtwoferro-magnetic end flanges connected by an external ferromagnetic yokesystem, a fixed ferro-magnetic core situated in thesaid-tubularelementso as to be partly in the opening enclosing the tubular part in one ofthe end flanges, a ferro-magnetic plunger core slidable in the tubularelement, a supporting element fixedly connected to said plunger core,two driving pawls, a drawing pawl-and a pushing pawl, both made ofresilient material and each of them fixedly supported at one end by thesaid supporting element,

a rotatable drum situated in direct line with the driving magnet andwith its axis of rotation traverse to the axis of said magnet, anendless belt, carried around and in driving connection with said drumand with straight parts having substantially the same direction asthe-axis of the driving magnet, a countersunk ratchet wheel with insideteeth in each ofthe two ends of the drum, each ratchet wheel cooperatingwith a separate driving pawl, apart at the free end of each driving pawlwith a direction substantially traverse to the longitudinal directionofsaid pawl, protruding into the countersunkratchet wheel with which thepawl cooperates and resting by the pawls own elastic-force on thetoothing of said wheel, either near 'one edge, or near both edges of thebelt a longitudinal track provided with different parts following eachother in the longitudinal direction of the track and some of which haveone given material thickness and others of which have another givenmaterial thickness, whichqmay be zero, a contact mechanism situated inthe space'enclosed by the endless belt, a number of contact devices insaid mechanism each provided with a part cooperating with a longitudinaltrack of the belt, at least one electrical contact in such acontact'device opened or closed by the device dependingon the materialthickness -of-the belt in the track'on the spot wheresthe said part ofthe contact device cooperates with the track.

3. An electromagnetic mechanism comprising a driving electromagnet witha winding enclosing a nonmagnetic tubular elementcarrying twoterm-magnetic end flanges connected byan external 'ferro-magnetic yokesystem, a fixed ferro-magnetic core situated in the said tubular elementso as to be partly in the opening enclos-' ing the tubular part in oneof the-end flanges, a ferromagnetic plunger core slidable in the tubularelement, a supporting element fixedly connected to said plunger core,two driving pawls, a drawing pawl and a pushing pawl, both made ofresilient material and each of them fixedly supported at one end by thesaidsupporting element,-a rotatable drum situated in line with thedriving magnet, and with its axis of rotation traverse to the axis ofsaid magnet, an endless belt with straightyparts having substantiallythe same direction asthe axis-ofsthe driving magnet, and carried aroundthe drum and in driving connection with this drum by means of'pins orsprockets on the surface of the drum cooperating with'openings in thebelt, a countersunk ratchet wheel with inside teeth in at least one ofthe two ends of the drum, a part at the free end of each driving pawlwith a direction substantially traverse to the longitudinal; directionof said :pawl, protruding into the countersunk ratchet wheel with whichthe pawl cooperates and resting 'by its own elastic. force on thetoothing of said wheel, either near one edgeor near both edges of thebelt a longitudinal track provided with different parts following eachother in the longitudinal direction of the track, and some of which haveone-given material thickness and others of which. have .7 r 13 anothergiven material thickness which may be zero, a contact mechanism situatedin the space enclosed by the endless belt, a number of contact devicesin said mechanism each provided with a part cooperating with alongitudinal track of the belt, at least one electrical contact in sucha contact device opened or closed by the device depending on thematerial thickness of the belt in the track on the spot where the saidpart of the device cooperates with the track.

4. An electromagnetic mechanism comprising a driving electromagnet witha winding enclosing a nonmagnetic tubular element carrying twoferro-magnetic end flanges connected by an external ferro-magnetic yokesystem, a fixed ferro-magnetic core situated in the said tubular elementso as to be partly in the opening enclosing the tubular part in one ofthe end flanges, a ferro-magnetic plunger core slidable in the tubularelement, a supporting element fixedly connected to said plunger core,two driving pawls, a drawing pawl and a pushing pawl, both made ofresilient material and each of them fixedly supported at one end by thesaid supporting element, a rotatable drum situated in line with thedriving magnet and with its axis of rotation traverse to the axis ofsaid magnet, an endless belt with straight parts havingsubstantially'the same direction as the axis of the driving magnet, andcarried around the drum and in driving connection with said drum bymeans of pins or sprockets on the surface of the drum cooperating withopenings in the said belt, a countersunk ratchet wheel with inside teethin each of the two ends of the drum, each ratchet wheel cooperating witha separate driving pawl, a part at the free end of each driving pawlwith a direction substantially traverse to the longitudinal direction ofsaid pawl, protruding into the countersunk ratchet wheel with which thepawl cooperates and resting by the pawls own elastic force on thetoothing of said wheel, either near one edge or near both edges of thebelt a longitudinal track provided with difiere'nt parts following eachother in the longitudinal direction of the track and some of which have.one given material thickness and others of which haveanother givenmaterial thickness which maybe Zero, a contact mechanism situated in thespace enclosed by the endless belt, a number of contact devices in 'saidmechanism each provided with a part cooperating with a longitudinaltrack of the belt, at least one electrical contact in such a contactdevice opened or closed by the device depending on the materialthickness of the belt in the track on the spot where the said part ofthe device cooperates with the track.

5. An electromagnetic mechanism comprising a driving electromagnet witha winding, enclosing a non-magnetic tubular element carrying twoferro-magnetic end flanges connected by an external ferro-magnetic yokesystem, a fixed ferro-magnetic core situated in the said tubular elementso as to be partly in the opening enclosing the tubular element in oneof the end flanges, a terro-magnetic plunger core slidable in thetubular element, a supporting element fixedly connected to said plungercore, two driving pawls, a drawing pawl and a pushing pawl, both made ofresilient material and each of them fixedlysupported at one end by thesaid supporting element, a rotatable drum situated in line with thedriving magnet and with its axis of rotation traverse to the axis ofsaid magnet. an endless belt, carried around and in driving connectionwith said drum and with straight parts having substantially the samedirection as the axis of the driving magnet, a countersunk ratchet wheelwith inside teeth in at least one of the two ends of the drum, a part atthe free end of each driving pawl with a direction substantially.traverse to the longitudinal direction of said pawl, protruding into thecountersunk ratchet wheel with which the pawl cooperates. and resting bythe pawls own elastic force on the toothing of said wheel, alongitudinal track near both edges of the belt, each provided withdifierent parts following each other in the longitudinal direction ofthe tracks and some of which have one given material thickness andothers another given material thickness which may be zero, a contactmechanism situated in the space enclosed by the endless belt, in saidcontact mechanism a number of contact devices arranged in a line, eachprovided with a part cooperating with the track near one edge of thebelt and one contact device with a part cooperating with the track nearthe other end of the belt, at least one electrical contact in eachcontact device opened or closed by this device depending upon thematerial thickness of the belt in the track on the spot where the saidpart of the contact device cooperates with the track, the parts withdifferent thickness in both tracks being arranged so as to cause thecontact devices arranged in a line to close and open their contactsaccording to a shifting binary code indicating the position reached bythe belt, and the other contact mechanism to close or open its contactso as to add such an element to the code produced by the other device,that either the total number of code-elements of one type is alwayseven, or the total number of elements of one type is always odd.

6. An electromagnetic mechanism comprising a driving electromagnet witha winding enclosing a non-magnetic tubular element carrynig twoferro-magnetic end flanges connected by an external ferro-magnetic yokesystem, a fixed ferro-magnetic core situated in the said tubular elementso as to be partly in the opening enclosing the tubular part in one ofthe end flanges, a ferro-magnetic plunger core slidable in the tubularelement, a supporting element fixedly connected to said plunger core,

two driving pawls, a drawing pawl and a pushing pawl,

magnet, an endless belt, carried around and in driving connection withsaid drum and with straight parts having substantially the samedirection as the axis of the driving magnet, a countersunk ratchet wheelwith inside teeth in each of the two ends of the drum, each ratchetwheel cooperating with a separate driving pawl, a part at the free endof each driving pa-wl with a direction substantially traverse to thelongitudinal direction of said pawl, protruding into the countersunkratchet wheel with which the pawl cooperates and resting by the pawlsown elastic force on the toothing of said wheel, a longitudinal tracknear both edges of the belt, each provided with different partsfollowing each other in the longitudinal direction of the tracks andsome of which have one given material thickness and others another givenmaterial thickness which may be zero, a contact mechanism situated inthe space enclosed by the endless belt, in said contact mechanism anumber of contact devices arranged in a line, each provided with a partcooperating with the track near one edge of the belt and one contactdevice with a part cooperating with the track near the other edge of thebelt, at least one electrical contact device opened or closed by thisdevice depending upon the material thickness of the belt in the track onthe spot where the said part of the contact device cooperates with thetrack, the parts with different thickness in both tracks being arrangedso as to cause the contact devices arranged in a line to close and opentheir contacts according to a shifting binary code indicating theposition reached by the belt and the other contact mechanism to close oropen its contacts so as to add such an element to the code produced bythe other devices, that either the total number of code-elements of onetype is always even, or the total number of elements of one type isalways odd.

7. An electromagnetic mechanism comprising a driving electromagnet witha winding enclosing a non-mag- -lar element so as to be partly in theopening enclosing the tubular part in one of the end flanges, aferro-magnetic plunger core slidable in the tubular element, asupporting element fixedly connected to said plunger core, two drivingpawls, a drawing pawl and a pushing pawl, both made of resilientmaterial and each of them fixedly supported at one end by the saidsupporting element, a rotatable drum situated in line with the drivingmagnet .and with its axis of rotation traverse to the axis of saidmagnet, an endless belt with straight parts having substantially thesame direction as the axis of the driving vmagnet, and carried aroundthe drum and in driving connection with this drum by means of pins orsprockets on the surface of the drum cooperating with openings in the.belt, a countersunk ratchet wheel with inside teeth in at least one ofthe two ends of the drum, a part at the free ,end of each driving pawlwith a direction substantially traverse to the longitudinal direction ofsaid pawl, protruding into the countersunk ratchet wheel with which thepawl cooperates and resting by the pawls own elastic force on thetoothing of said wheel, a longitudinal track near both edges of thebelt, each provided with difierent parts following each other in thelongitudinal direction of the tracks and some-of which have one givenmaterial thickness and others another given material thickness which maybe zero, a contact mechanism situated in the .space enclosed by theendless belt, in said contact mechanism a number of contact devicesarranged in'a line, each provided with a part cooperating with the tracknear one edge of the belt and one contact device with apart cooperatingwith the track near the other edge of the belt, at least one electricalcontact in each contact device opened or closed by this device dependingupon the material thickness of the belt in the track on the spot wherethe said part of the contact device cooperates with the track, the partswith different thickness in both tracks being arranged so as to causethe contact devices arranged in a line to close and open their contactsaccording to a shifting binary code indicating the position reached bythe belt, and the other contact mechanism to close or open its contactsso as to add such an element to the system, a fixed ferro-magnetic coresituated in the said tubular element so as to be partly in the opening,enclosing the tubular part in one of the end flanges, a ferromagneticplunger core slidable in the tubular element, a

supporting element fixedly connected to said plunger core, two drivingpawls, a drawing pawl and a pushing pawl, both made of resilientmaterial and each of them fixedly supported at one end by the saidsupporting element, a rotatable drum situated in line with the drivingmagnet and with its axis of rotation traverse to the axis of saidmagnet, an endless belt with straight parts having substantially thesame direction as the axis of said driving magnet, and carried aroundthe drum and in driving connection with said drum by means of pins orsprockets on the surface of the drum cooperating with openings in thesaid belt, a countersunk ratchet wheel with inside teeth in each of thetwo ends of the drum, each ratchet wheel cooperating with a separatedriving pawl, a part at the free end of each driving pawl with adirection substantially traverse to the longitudinal direction of saidpawl, protruding into the countersunk ratchet wheel with which the pawlcooperates and resting by the pawls own elastic force on the toothing ofsaid wheel, a longitudinal track near both edges of the belt, eachprovided with diiferent parts following each other in the longitudinaldirection of the tracks and some of which have one given materialthickness and others another given material thickness which may be zero,a contact mechanism situated in the space enclosed by the endless belt,in said contact mechanism anumber of contact devices arranged in a line,each provided with a part cooperating with the track nearone edge of thebelt and one contact device with a part cooperating with the track nearthe other end of the belt, at least one electrical contact in eachcontact device opened or closed by this device depending upon thematerial thickness of the belt in the track on the spot where the saidpart of the contact device cooperates with the track, the parts withdifferent thickness in both tracks being arranged so as to cause thecontact a brush with its free end on a track of the belt and a contactstrip arranged at the side of the belt opposite to the brushesin such aposition, that a brush establishes electrical contact with the strip ifa part of the track in which the material thickness iszero is under thebrush.

10. In an electro-magnetic mechanism according to claim 1, in eachcontact device inthe contact mechanism a brush with its free end on "atrack ofthe belt and at the side of the belt opposite to the brushes,opposite to each brush a contact piece in such a position that saidbrush establishes an electrical contact with the said con- .tact pieceit a part of the track in which the material thickness is zero is underthe brush.

11. In an electro-magnetic mechanism according to claim 1 in eachcontact device a lever, rotatable around an axis and an elasticelementexerting a force on said lever so asto cause one end of the leverto rest against the material of the track, and at least one contactelement arranged in the contact device in such a position in thevicinity of thelever, that this lever establishes electrical contactwith such a contact element in one of the two positions in which itcan'bebrought by the track of the belt.

vl,'Z. Electro-magnetic mechanism according to claim 1, in which eachdrivingpawl remains atone sideof the symmetry plane through the axis ofrotation of the drum and perpendicular to the direction of motion of thepawls whilst performing its stroke, the stroke during which the drum isdriven by a pawl beginning at that end of the stroke which is nearest tothe said symmetry plane.

13. In an electro-magnetic mechanism according to claim 1, furthercomprising a stop for a driving pawl arranged to press the said pawlinto the toothing of the ratchet at the end of the stroke during whichthe pawl drives the ratchet.

14. In an electro-magnetic mechanism according to claim 1, furthercomprising a lengthenedhub arranged on the drum in a countersunk ratchetwheel and a bent part of a pawl cooperating with said ratchet wheelarranged to run against the lengthened hub at the end of the strokeduring which the pawl drives the ratchet wheel thereby causing the pawlto be pressed in the toothing of the ratchet wheel.

15. In an electro-magnetic mechanism according to claim 1, furthercomprising two recesses in the supporting element, and two driving pawlseach of them fixedly supported at one end by the said supporting elementat the end of a recess away from the rotatable drum. and passing throughand being able .to switch freely in the said recess.

16. In an electro-magnetic mechanism according to claim 1, furthercomprising a second fixed core situated in the tubular element so as tobe partly in theopening amuse v 17 1'3 enclosing the tubular element inthe secondlend flange, References Cited in the fileof this patent aferro-magnetic plunger core slidable 'in the tubular element between thetwo fixed cores and a connecting ele- UNITED STATES PATENTS mentconnecting the plunger core with the supporting 615,418 Wilkinson Dec.6, 1898 element running through a bore of one of the fixed cores. 5

17. In an eiectro-magnetic element according to claim FOREIGN PATENTS 1,further comprising a layer of polyamide covering the slidable com372,535 Germany Mar. 29, 1923

